Magnetoresistive sensors responsive to changes caused by magnetic fields are increasingly employable as read transducers in smaller sized magnetic disk drive heads having decreased disc surface velocity because the resistivity changes depend on magnetic flux, not disk speed, and because sensor output may be scaled by the sense current.
These sensors usually comprise a thin strip of ferromagnetic alloy of low coercivity, such as NiFe (Permalloy), magnetized along an easy axis direction. Such magnetized strips are typically mounted in the head so that their easy axis is transverse to the direction of disk rotation and parallel to the plane of the disk. Magnetic flux from the disk causes rotation of the magnetization vector of the strip, which in turn causes a change in resistivity which can be detected by a sense current flowing between lateral contacts. The resistivity varies according to the cosine-square of the angle between the magnetization vector and the current vector. If the magnetization and current vectors are aligned, the initial change in resistivity due to disk magnetic flux is low and unidirectional. Typically, either the easy axis magnetization vector or the current vector is biased to approximately 45 degrees to increase responsiveness to angular change in the magnetization vector and to linearize the sensor output.
One problem encountered with MR sensors is Barkhausen noise caused by irreversible motion of magnetic domains in the presence of an applied field, i.e. coherent rotation of the magnetization vector is non-uniform or instead depends upon domain wall behavior. This noise mechanism is eliminated by creating a single magnetic domain in the easy axis direction of the strip.
Many different means have been employed to both linearize the sensor output and to provide for a single domain in the sense region. An increase in the length of a magnetoresistive strip relative to its height is known to contribute to formation of a single magnetic domain in the central portions of the strip. Multiple closure domains occur at the ends of long strips. These migrate toward the center under the influence of external fields. Long strips, however, may be subject to cross-talk in lateral portions of the strip and may conduct magnetic flux from adjacent tracks into the sense region of the strip. Short strips, in contrast, spontaneously "fracture" into multiple domains.
Efforts have been made to provide single domains in the sensor region by shaping the strip so as to reduce edge demagnetizing fields while providing a relatively short physical dimension in the sensor region. Even shaped strips "fracture" into multiple domains in the presence of strong transverse magnetic fields caused by the inductive write field. Soft-magnetic shields are conventionally employed to isolate the sensor from magnetic fields produced by the inductive writer or external fields.
The formation of single domains has been achieved by providing a longitudinal magnetic field in strips. Such a magnetic field has to be strong enough to cause the formation of a relatively stable, single domain in the central sensor region. This initialization field can be provided by a barber pole, which is also used to cant the direction of the sense current relative to the easy axis magnetic vector.
For short strips, efforts have been made to maintain single domains by permanent longitudinal biasing from adjacent external permanent magnets or exchange coupled antiferromagnetic or permanent magnet material which forms a coupled film with the sensor.
Exchange or permanent magnet biasing is commonly employed. However, antiferromagnetic materials at an exposed interface can result in destructive corrosion and require a protection layer. Further, because exchange biasing is a quantum-mechanical interaction effect, reliable atomic interaction is required. Such processing is difficult, but with care can be achieved with good yields. The antiferromagnetic exchange effect is substantially reduced in typical disk drive operating environments due to a strong temperature dependence.